The present invention relates to improvements to photoelectric direction finders in which the modulating and spatial filtering functions due to movable reticles are separate from the deviation measurement function.
A system of this type is described in U.S. Pat. application Ser. No. 19697. The reticles are such that there is a modulation of the reception beam at a desired frequency. A four quadrant detector is used. An intermediate optical means, called the deviation measurement means, are placed between the reticles located in the focal plane of the reception optical means and the detector in such a way as to form an image spot of a certain diameter on the photosensitive detection plane. This arrangement makes it possible to mechanically decouple the reticles--reception optical means assembly from the optical deviation measurement--detector assembly located downstream. More particularly, through using a deviation measurement optical means of the half-ball type, whose centre of curvature essentially coincides with the centre of the four quadrant detector, it is possible to produce a relative movement of the reticles with respect to the detector by rotation about the said centre. This system is advantageously applied to the construction of a passive infrared homing apparatus.